Virtually all existing high energy (>few MeV) linac-driven FELs compress bunch length through use of off-crest acceleration on the rising side of the RF waveform (to induce an “energy chirp”) followed by transport through a magnetic chicane. Some implementations use this method in conjunction with velocity compression of the low energy beam in the front end of the system. Though effective to some degree, this approach has at least three flaws: 1) it is difficult to correct aberration effects, particularly phase space distortion due to RF curvature. Typically harmonic RF is invoked in response, at considerable expense—and with the numerous difficulties attendant the use of high frequency resonant cavities; 2) acceleration on the rising side of the RF waveform exacerbates some aspects of longitudinal space charge (LSC) induced degradation of beam quality, resulting in reduced peak current when interacting with coherent synchrotron radiation (CSR) effects; and 3) chicanes, and all other achromatic “negative compaction” bunch length compressors (those with R56<0, in which higher energy electrons travel a shorter distance than lower energy ones) necessarily create a parasitic compression of the bunch during the final compression process and expose the (somewhat over-compressed) bunch to interaction with coherent transition radiation from the end of the penultimate dipole of the compressor—exacerbating beam quality degradation from CSR.
Accordingly, it would be advantageous to provide a bunch length compression method for free electron lasers that avoids aberration effects, eliminates LSC induced degradation of beam quality, and avoids parasitic compressions of the bunch during the final compression process.